Chemical-equilibrium problems are among the most important and at the same time most complex and difficult general-chemistry problems. In this paper, errors made by the students, while solving such problems, are discussed. These errors are of two kinds: (i) Β‘randomΒ’ errors caused by hastiness, or by thoughtlessness, or by an overload of working memory, or by field dependence or a combination of the above factors. (ii) Β‘systematicΒ’ errors that are caused by misconception(s) or by a difficulty in understanding of the underlying theory, concepts, or processes. Students in their final upper-secondary school year (age 17-18), attending an elective chemistry course, participated (N = 120-148, depending on the question). The errors that were detected were categorised into five categories: (1) the equilibrium constant; (2) stoichiometry; (3) heterogeneous equilibria; (4) the direction of a reaction which is not at equilibrium (the disturbance of the chemical equilibrium); (5) gas equilibria and the ideal-gas law.
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